Para retomar os downloads que são interrompidos ao usar o Firefox você deve considerar o uso de Faça o download via IDM (Internet Download Manager) que funciona em conjunto com o Internet Download Manager .
Internet Download Manager (IDM) is a tool to increase download speeds by up to 5 times, resume and schedule downloads. Comprehensive error recovery and resume capability will restart broken or interrupted downloads due to lost connections, network problems, computer shutdowns, or unexpected power outages.